1. Field of the Invention
The present invention relates to an ion source and more particularly to an electron impact ion source.
2. Description of the Prior Art
Ion implantation has been a key technology in semiconductor device manufacturing for more than twenty years, and is currently used to fabricate the p-n junctions in transistors, particularly in CMOS devices, such as memory and logic chips. By creating positively-charged ions containing various dopant elements, such as, 75As, 11B, 115In, 31P, or 121Sb, required for fabricating the transistors in, for example, silicon substrates, known ion implanters can selectively control both the energy (hence implantation depth) and ion current (hence dose) of ions introduced into transistor structures. Ion implanters have traditionally used ion sources which generate ribbon beams of up to about 50 mm in length. These beams are transported to the substrate at a predetermined uniform dose by electromagnetic scanning of the beam across the substrate, mechanical scanning of the substrate across the beam, or both.
So-called medium current implanters typically incorporate a serial (one wafer at a time) process chamber, which offers high tilt capability (e.g., up to 60 degrees from substrate normal). The ion beam is typically electromagnetically scanned across the wafer, in an orthogonal direction to ensure dose uniformity. In order to meet implant dose uniformity and repeatability requirements which typically allow only a few per cent variance in these quantities, the ion beam should have excellent angular and spatial uniformity (angular uniformity of beam on wafer of  less than 2deg, for example). The production of beams possessing these characteristics imposes severe constraints on the beam transport optics of the implanter, and the common place use of large-emittance plasma-based ion sources often results in increased beam diameter and beam angular divergence, causing beam loss during transport due to vignetting of the beam by the various apertures present within the beam line of the implanter. Currently, the generation of high current ( greater than 1 mA) ion beams at low ( less than 5 keV) energy is problematic in serial implanters, such that wafer throughput is unacceptably low for certain low-energy implants (for example, in the creation of source and drain structures in leading-edge CMOS processes). Similar transport problems also exist for batch implanters (processing many wafers mounted on a spinning disk) at the low beam energies of  less than 5 keV per ion.
While it is possible to design beam transport optics which are nearly aberration-free, the ion beam characteristics (spatial extent, spatial uniformity, angular divergence and angular uniformity) are nonetheless largely determined by the emittance properties of the ion source itself (i.e., the beam properties at ion extraction which determine the extent to which the implanter optics can focus and control the beam as emitted from the ion source). Arc-discharge plasma sources currently in use have poor emittance, and therefore severely limit the ability of ion implanters to produce well-focused, collimated, and controllable ion beams. Thus, there is a need for an ion source for use in a semiconductor manufacturing which provides a well-focused, collimated and controllable ion beam.
Briefly, the present invention relates to an ion source configured for integration into both existing ion implanters used in semiconductor manufacturing and emerging ion implantation platforms, and is also suitable for use in ion dosing systems used in the processing of flat panel displays. The ion source in accordance with the present invention includes the following features, all of which depart from the prior art to produce a well-focused, collimated and controllable ion beam:
Ionizing electron beams generated external to the ionization chamber, thereby extending the emitter lifetime.
90 degree magnetic deflection of electron beams such that no line-of-sight exists between the emitter and the process gas load, and the emitter is protected from bombardment by energetic charged particles.
Two opposed electron beams which can be operated simultaneously or separately.
Use of a deceleration lens to adjust the ionization energy of the electron beam, substantially without affecting electron beam generation and deflection.